Recently, an absorbent resin is used extensively in sanitary goods, such as paper diapers, sanitary napkins, and incontinence pads, to absorb body fluids. Besides the sanitary goods, the absorbent resin is also used extensively in water-retaining materials for soil to keep water in soil or drip absorbing materials to absorb drip from foods or the like.
Examples of such absorbent resins include: a partially neutralized crosslinked polymer of polyacrylic acid, a hydrolyzed graft polymer of starch-acrylonitrile, a neutralized graft polymer of starch-acrylic acid, a saponified copolymer of vinyl acetate-acrylic ester, a hydrolyzed copolymer of acrylonitrile or acrylamide, or a crosslinked product thereof, and a crosslinked polymer of a cationic monomer.
Notable properties of the absorbent resin include absorbency and absorbing rate when brought into contact with an aqueous liquid like a body fluid, liquid permeability, gel strength of swelled gel, and absorbing power of absorbing water from a base material containing an aqueous liquid, etc. However, each property is not necessarily correlated positively to one another. For example, the higher the absorbency, the lower the absorbing rate, liquid permeability, and gel strength. To solve the above problem, a crosslinking treatment (secondary crosslinking treatment) using a crosslinking agent (denaturant) is conventionally applied to the absorbent resin to well balance (improve) these properties and enhance the absorbing property.
A secondary crosslinking structure is introduced into the absorbent resin in the following manner: a swelling liquid in which the absorbent resin is swelled with a solvent, or a dispersing liquid in which the absorbent resin is dispersed in a dispersing medium is prepared, and a crosslinking agent or a solution thereof is added to the above prepared liquid and mixed with each other, so that the reaction of the absorbent resin and crosslinking agent takes place in a so-called solid-liquid system. The solvent or dispersing medium referred to herein is, for example, a hydrophilic compound, such as water and alcohol.
Examples of the above method of introducing the secondary crosslinking structure into the absorbent resin using a crosslinking agent include: a method of using polyhydric alcohol (Japanese Laid-open Patent Application No. 180233/1983 (Tokukaisho No. 58-180233) and Japanese Laid-open Patent Application No. 16903/1986 (Tokukaisho No. 61-16903)), a method of using a polyglycidyl compound, a polyaziridine compound, a polyamine, or polyisocyanate (Japanese Laid-open Patent Application No. 189103/1984 (Tokukaisho No. 59-189103)), a method of using glyoxal (Japanese Laid-open Patent Application No. 117393/1977 (Tokukaisho No. 52-117393)), a method of using polyvalent metal compound (Japanese Laid-open Patent Application No. 136588/1976 (Tokukaisho No. 51-136588), Japanese Laid-open Patent Application No. 257235/1986 (Tokukaisho No. 61-257235), and Japanese Laid-open Patent Application No. 7745/1987 (Tokukaisho No. 62-7745)), a method of using a silane coupling agent (Japanese Laid-open Patent Application No. 211305/1986 (Tokukaisho No. 61-211305), Japanese Laid-open Patent Application No. 252212/1986 (Tokukaisho No. 61-252212), and Japanese Laid-open Patent Application No. 264006/1986 (Tokukaisho No. 61-264006)), a method of using an epoxy compound and a hydroxy compound (Japanese Laid-open Patent Application No. 132103/1990 (Tokukaihei No. 2-132103)), a method of using alkylene carbonate (German Patent No. 4,020,780), etc.
Also, a variety of methods are proposed to distribute the crosslinking agent over the surface of the absorbent resin more evenly to crosslink near the surface of the absorbent resin uniformly. Examples of such methods are: a method of adding the crosslinking agent to the absorbent resin in the presence of inactive inorganic powders (Japanese Laid-open Patent Application No. 163956/1985 (Tokukaisho No. 60-163956) and (Japanese Laid-open Patent Application No. 255814/1985 (Tokukaisho No. 60-255814)), a method of adding the crosslinking agent to the absorbent resin in the presence of dihydric alcohol (Japanese Laid-open Patent Application No. 292004/1989 (Tokukaihei No. 1-292004)), a method of adding the crosslinking agent to the absorbent resin in the presence of an ether compound (Japanese Laid-open Patent Application No. 153903/1990 (Tokukaihei No. 2-153903)), a method of adding the crosslinking agent to the absorbent resin in the presence of an alkylene oxide adduct of monohydric alcohol, an organic acid salt, or lactam (European Patent No. 555,692), etc.
However, the above conventional methods have the following problems. That is, because the conventional methods use the solvent or dispersing medium to react the absorbent resin with the crosslinking agent, these methods must include a post-treatment process, such as a removing step for removing the solvent or dispersing medium and a drying step. This is the reason why the entire procedure of introducing the secondary crosslinking structure into the absorbent resin is complicated. In addition, the crosslinking agent and the solvent or dispersing medium may reside in the post-reaction absorbent resin, thereby possibly making the resulting absorbent resin unsafe. Further, it is quite difficult to remove or collect an excessive crosslinking agent when the reaction ends.
Moreover, when microscopic powders of the absorbent resin are used, for example, a mixture of the absorbent resin and the solvent or dispersing medium produces an agglomerate, thereby making it impossible to swell or disperse the absorbent resin uniformly in a satisfactory manner. Thus, the secondary crosslinking structure can not be introduced into all kinds of absorbent resin uniformly because of their shape or size. In addition, the swelling liquid or dispersing liquid must be stirred relatively hard to let the absorbent resin swell or disperse the same uniformly in a satisfactory manner. Thus, the absorbent resin is susceptible to a physical damage, and for example, the surface of the absorbent resin is often damaged when the reaction ends. Note that the agglomerate referred to herein means masses of agglomerated particles.
Also, to carry out the crosslinking treatment at a relatively high degree, for example, to attain high crosslinking density and depth, a relatively large amount of solvent or dispersing medium must be used, which makes the reaction of the absorbent resin and crosslinking agent inefficient. In addition, when a relatively large quantity of the solvent or dispersing medium is used, not only the agglomerate is readily produced, but also the energy cost of the post-treatment process increases undesirably.
Further, the conventional methods can change (improve) the balance of the notable properties of the absorbent resin to a certain extent, but not beyond the extent of practical use. For example, the recent sanitary goods tend to use more amount of absorbent resin while reducing its thickness. However, if the desired properties for the absorbent resin used in the materials of the sanitary goods, that is, an absorbent material, the conventional methods can not balance the properties in a practical manner. Therefore, there has been an increasing need for a method of balancing the properties in a practical manner, in other words, a method of further improving the quality of the absorbent resin.
More specifically, in case of an absorbent material containing a large amount of absorbent resin, namely, having a high concentration of absorbent resin, the desired properties are the absorbing property under pressure, such as absorbency and water retaining ability under pressure, bonding and shape-keeping properties when water is absorbed into spaces among the particles of the absorbent resin which are disclosed in Japanese Laid-open Patent Application No. 96159/1993 (Tokukaihei No. 5-96159), and bonding and shape-keeping properties of an absorbent material made of the absorbent resin, cellulose fiber, etc. when the absorbent material has absorbed water. However, the conventional methods are not effective enough to further improve the absorbing property under pressure. In addition, when the method disclosed in above Japanese Laid-open Patent Application No. 96159/1993 (Tokukaihei No. 5-96159) is adopted, although it becomes possible to control the inconveniences, for example, the absorbent resin is released from the absorbent material or migrates within the absorbent material while water is being absorbed, the absorbency under pressure can be hardly improved in some kinds of absorbent resin. Further, once the absorbent resin is produced, its bonding property deteriorates over time before the absorbent resin is actually used.
Therefore, the conventional methods cause a number of problems specified as above in a reaction of the absorbent resin and crosslinking agent, namely, in a reaction of a hydrophilic polymer and a denaturant.